Instruction Sets:Addressing modes and

FormatsGroup #4

Eloy Reyes Rafael Arevalo Julio Hernandez Humood Aljassar

Computer DesignEEL 4709cProf: Herman WatsonSpring 2009

Addressing Modes

Immediate Direct Indirect Register Register Indirect Displacement (Indexed) Stack

Immediate Addressing Operand is part of instruction Value to be loaded is fixed at compile-time

(not very flexible) e.g. ADD 8

Add 8 to contents of accumulator 8 is operand

Does not have an effective address Fast (Value to be loaded – included in inst) Value stored in memory follows

immediately the operation code in memory

Immediate Addressing Diagram

OperandOpcode

Instruction

Direct Addressing

Address field contains address of operand

Effective address (EA) = address field (A) e.g. ADD C

Add contents of cell C to accumulator Look in memory at address C for operand

Directly retrieves stored value in memory from another memory location.

Fast (Internal RAM) Limited address space

Direct Addressing Diagram

Address AOpcode

Instruction

Memory

Operand

Indirect Addressing

Memory cell pointed to by address field contains the address of (pointer to) the operand

EA = (C) Look in C, find address (C) and look there

for operand Exceptional level of flexibility. Always refers to Internal RAM.

Indirect Addressing

2n where n = word length Multilevel, cascaded, or nested Address space (Large) Slower Multiple memory accesses -> operand

Indirect Addressing Diagram

Address AOpcode

Instruction

Memory

Operand

Pointer to operand

Register Addressing

Operand contained in register (Named -> address filed)

Registers (Limited) Needs -> Very small address field

Instructions (Fast, hence faster instruction fetch)

EA= R

Register Addressing

Address space -> Very limited Memory Address -> None Multiple registers -> Improves

performance N.B. C programming Good compiler writing or assembly

programming -> Required Execution -> Very fast

Register Addressing Diagram

Register Address ROpcode

Instruction

Registers

Operand

Register Indirect Addressing

Address of operand obtained indirectly via an address register.

Indirect addressing uses an address register to point at the location of the operand in memory.

EA = (R) Generates lots of different addresses

when program executed. Address Space -> Large (2n)

Register Indirect Addressing Diagram

Register Address ROpcode

Instruction

Memory

OperandPointer to Operand

Registers

Displacement Addressing

Choosing displacement field size is important -> directly affects instruction length.

EA = A + (R) Address field hold two values

A = base value R = register that holds displacement or vice versa

Displacement Addressing Diagram

Register ROpcode

Instruction

Memory

OperandPointer to Operand

Registers

Address A

+

Relative Addressing

An address specified by indicating its distance from another address. (Base Address)

A version of displacement addressing For example: B+15 B being the base address and 15 the

distance (Called the offset)

Base-Register Addressing

R -> Explicit or Implicit R contains pointer to base address A holds displacement e.g. segment registers in 80x86

Indexed Addressing

Variables which use indexed addressing: Variables referenced by pointers Arrays Local Variables, in stack-based languages.

E.g. (IX, IY registers) A = base R = displacement EA = A + R Automatically increments address with

the value stored in an index register.

Combinations

Combinations of the three basic addressing modes -> additional addressing modes.

Resultant or operational instruction cannot be immediate data. (Direct of Indirect memory location)

Postindex EA = (A) + (R)

Preindex EA = (A+(R))

Stack Addressing

Data structure grows downward from high to low memory in SP.

New byte pushed onto the stack -> SP decremented

Byte pulled from the stack -> SP incremented

Operand is (implicitly) on top of stack e.g.

ADD Pop top two items from stackand add

Pentium Addressing Modes

Virtual or effective address is offset into segment Starting address plus offset gives linear address This goes through page translation if paging enabled

12 addressing modes available Immediate Register operand Displacement Base Base with displacement Scaled index with displacement Base with index and displacement Base scaled index with displacement Relative

Pentium Addressing Mode Calculation

PowerPC Addressing Modes

Load/store architecture Indirect

Instruction includes 16 bit displacement to be added to base register (may be GP register)

Can replace base register content with new address Indirect indexed

Instruction references base register and index register (both may be GP)

EA is sum of contents Branch address

Absolute Relative Indirect

Arithmetic Operands in registers or part of instruction Floating point is register only

PowerPC Memory Operand Addressing Modes

Instruction Formats

Essentially the layout of the Bits in an Instruction set including: OpCodes Operand(s) expressed either implicitly or

explicitly Memory address Register

Usually more than one instruction format is used and usually varies depending on whether its addressing Memory, I/O devices or a Register

Instruction Length

Affected by and affects: Memory size Memory organization Bus structure CPU complexity CPU speed

Longer Instruction Length

Pros: More OpCodes Operands Addressing Modes Greater Address Range Therefore easier

Programming Language Cons:

Waste of Space

Example: A 64 bit instruction is less than twice as useful as a 32 bit instruction.

Allocation of Bits

Number of addressing modes: i.e. certain opcodes always call for indexing, in others, addressing

modes must be explicit and more than one bit might be needed. Number of operands:

Typical instructions in today’s machines call for two operands. Register versus memory:

The more registers that can be used for oprand references , the fewer bits are needed. Most resent day designs have at least 32 registers.

Number of register sets: A method used by Pentium uses two sets of 8 registers and only 3

bits are required to identify the register. The opcode will determine which register is being referenced.

Address range: Rarely used because it imposes a limitation in the amount of

address since its directly related to the size of the address.

PDP-8 Instruction Design

Only 12 bit Instructions 7 Bits to Reference to Memory:

(2)^7 = 128 Different Words

2 Bits used as modifiers ; First referenced to page

containing instruction Second used to indicate

whether direct or indirect addressing is to be used.

3 Bits used for the opcode: Of the 8 different opcode

combinations, 2 are used to reference a page bit or an indirect leaving 6 basic operations.

PDP-8 Instruction Format

PDP-10 Instruction Format

• Orthogonality• Principle that two variables are independent of

each other.• Completeness

• Each arithmetic data type (integer, fixed-point, real) should have a complete and identical set of operations.

• Direct addressing• Used instead of base plus displacement

addressing to lower the burden on the programmer.

PDP-11 Instruction Format

• The PDP-11 was designed to provide a powerful and flexible instruction set

• Only constraint was the use of a 16-bit minicomputer [BELL70]

• It employs 16-bit general purpose registers• Two of these 16 have a significant purposes:

• One is used as a stack pointer for special purpose stack operations

• The other is used as the program counter (containing address of next instruction)

VAX Instruction Examples

VAX

Questions?

Why is it known immediate addressing? Which addressing mode has a limited address space? How does data structure grows in stack addressing? Give two examples of indexed addressing modes. Why do programmers prefer longer instruction ? How many bits does the PDP-8 design use for addressing? How is called the address the Pentium translation mechanism? In register operand mode, what one has to with 64-bit floating

point operation? The PDP-10 was designed to be what kind of system and with

what emphasis? What two criteria were used in designing the VAX instruction

format?